amp reading on ground wire

[jg3317]

Transformer is a 480/277vac 2500kva transformer at a factory.

Feeds two different switchgears.

1'st switchgear is a 3 phase/4 wire switchgear with 1200 main breaker
(3)500mcm per phase and (3) 250mcm neut connections

2nd switchgear is a 3 phase/3 wire switchgear with 1200 main breaker
(3)500mcm per phase and (3) 250mcm ground connections


I haven't completed complete documentation of loads from each switchgear. Customer states that they only utilize 480vac load, No 277vac utilized.
When documenting one line diagram today I clamped a amp meter on the neutral buss connection on

#1 switchgear and I had 30 amps of current on the neutrals to the transformer.

#2 switchgear had 15 amps on the ground wires to the transformer

If the customer is correct and they are not utilizing any of the 277vac load, then what would explain the amp readings on the neutral and ground cables??

The reason we were asked to look at this installation, customer states they are loosing motors and having other problems that tech supports ask if equipment is grounded and customer says yes but are wondering how good of a ground system they actually have.

 

[EEPeder]

If there is no intentional path from phase to neutral of the transformer, i.e. 277V load (and I definitely wouldn't take the customer's word for that), then there is an unintentional path, i.e. a ground fault. You can see small currents (less than 1A) due to capacitive paths but definitely not these magnitudes. A ground fault has to be fairly high impedance (probably inadequate bonding) if the current is only 15-20A. There should definitely not be current on the ground wires from the 3-wire panel, so there is definitely a problem here. A neutral to ground connection at the 4 wire panel plus 277V load on the 4wire wire problem might cause this. This is the least bad scenario I can imagine.

What is the total current on the transformer neutral bushing?

Can you clamp the service conductors to each of the panels to determine net current? Can you clamp the feeder conductors from each of the panels to determine net current?

 

[Pierre C Belarge]

If the transformer is Utility supplied, this makes the two switchgears services. This means you would be required to installed the grounded (neutral conductor) from the transformer to the the gear. 250.24(C). Lets not forget 230.95 either.

 

[ramsy]

Re: amp reading on ground wire

#1 switchgear had 30 amps of current on the neutrals to the transformer.
#2 switchgear had 15 amps on the ground wires (from) the (same) transformer

Could that 30A neutral imbalance be disributed back as 15A on gear #2, thru the shared xfmr and common bondings. Harmonics are known to circulate thru systems this way.

A meter with a Hz funciton will show if #1 neutral and #2 ground has 180Hz as the predominant magnitude, rather than the 60Hz fundamental.

 

[jg3317]

We have set up with power company to open transformer and inspect connections and take readings at the transformer I'll post our findings Friday.

Jim

 

[ramsy]

I'll post our findings Friday.

Since the 500mcm x (3) can't be derated anywhere near 0.80 for 1200A, the current-carrying neutral makes the installation to gear #1 a violation of 310-15(B)2a & 4c. Don points out an exception if this is a service with more than one service disconenct, and was calculated for load rather than total of the service OCPD's.

 

[don_resqcapt19]

Roger,

Since the 500mcm x (3) can't be derated anywhere near 0.80 for 1200A, the current-carrying neutral makes the installation to gear #1 a violation of 310-15(B)2a & 4c.

Just because the neutral is actually carrying current does not make it a "current carrying conductor" for derating purposes. In this case it would only be a current carrying conductor if the majority of the load was non-linear.
Don

 

[ramsy]

OK, I see neutrals that only carry circuit imbalance don't count as current-carrying conductors, except where a phase is missing from a 4-wire-Y circuit, or a major portion of nonlinear load causes neutral harmonics. 310-15(B)4a-c.

Don, many thanks for that correction.

 

[jg3317]

Update to original question

There are two 2500kva transformers for service to factory

Transformer #1 has a ground rod in the primary cabinet and it is connected to the transformer case and to the XO terminal

Transformer #2 has a ground rod in the primary cabinet and it is connected to the transformer case and to the XO terminal


These transformers are installed side by side and the two ground rods are not connected to each other they are driven into the ground approx 10 feet from each other and neither transformer is connected to any other type of a ground ring. This building consist of eight different building that were built from the 1800's, 1950, 60's 70, 80's. To the knowledge of the engineering department there is not type of grounding to the building grounding ring to bond building columns.

Transformer #1 on feeds two 1200 amp switchgears. (Switchgear #1 and Switchgear #2)

Transformer #2 feeds a 1200 amp switchgear and a 3000 amp switchgear: (switchgear #3, Switchgear #4).

Switchgear #1 doesn't have bonding jumper installed from neutral bar to ground bar and there are loads utilizing the grounding bar that isn't bonded to anything.

Switchgear #2, As stated previously facility only utilizes 480vac 3 phase circuits as far as I know. I clamped amprobe (super) on the three neutral cables connected the the neutral bar that does have the bonding jumper installed. I had amp readings of 30amps, 20 amps and 6 amps respectively on each of the three neutral cables. I also clamped a Greenlee True RMS digital amp meter on cables and received same readings. We removed two of the three cables from the neutral bar and there was an very light spark when I removed the cables. I check for voltage and had .0067 ac volts on one cable and .0089 volts on second cable to the neutral buss. I also placed a amp meter on the third cable still connected to the neutral bus and I had a reading of 9 amps.
I also connected my digital meter in series with the two cables I removed to check amperage and got a reading of 0 amps from cables to neutral bar.

I will have to admit I was and still am confused by the readings I received. I took my digital meter that has Hertz reading and placed meter between to cables I removed and the neutral bar and had a reading of 147 Hertz and was not a steading reading jumped from about 130 to 180 hertz.

Breaker #2 in Switchgear #2 is a 600 amp breaker that feeds a large Gardner Denver air compressor, there are three 500mcm feeder cables for the 3 phase and a #4 ground wire. The air compressor amp load is 200 amps when it was running, I clamped amprobe on ground cable at the local disconnect by the air compressor and had a reading of 5 amps.

I cannot explain this



Switchgear #3 has four cables connected to the neutral bus.
one to the building column, which shows 12 amps on it when I place amp probe on cable? I never removed cable from neutral bar for any further testing. Basically I was getting same results as in switchgear #2, amp probes show amperage but when I remove cable it doesn't increase on other cables as I would think it would.

Can anyone explain what is occurring? Harmonics?

 

[George Stolz]

Switchgear #1 doesn't have bonding jumper installed from neutral bar to ground bar...

Is there a neutral brought in from the transformer? It should be bonded from neutral to ground at this service disconnect.

and there are loads utilizing the grounding bar that isn't bonded to anything.

Are you saying there are loads using the grounding conductors for neutral current? Sounds like it's contributing (if not all of) your problem.

Switchgear #2, ... I had amp readings of 30amps, 20 amps and 6 amps respectively on each of the three neutral cables.

What are these neutral conductors you're disconnecting? Line side of the service or load side? Are they in parallel, or going to different destinations?

We removed two of the three cables from the neutral bar and there was an very light spark when I removed the cables. I check for voltage and had .0067 ac volts on one cable and .0089 volts on second cable to the neutral buss.

If these are conductors in parallel on the line side of the service, these readings make some sense.

I would begin by getting all the neutrals and grounding conductors bonded at the services, and see what transpires.

But I am only thinking out loud, wait for qualified responses.

 

[catchtwentytwo]

These transformers are installed side by side and the two ground rods are not connected to each other they are driven into the ground approx 10 feet from each other and neither transformer is connected to any other type of a ground ring. This building consist of eight different building that were built from the 1800's, 1950, 60's 70, 80's. To the knowledge of the engineering department there is not type of grounding to the building grounding ring to bond building columns.

Sound like one ground is better than the other. By the way, is there any permanently installed metering (CT's or PT's) in the switchgear(s)? Any instrument transformers?

 

[ramsy]

..Hertz reading ..neutral bar and had a reading of 147 Hertz and was not a steading reading jumped from about 130 to 180 hertz.

AKA triplen or Zero-sum harmonics. Meter reads the highest magnitude, so congratulations, you caught the big one without buying a costly analyzer.

..I clamped amprobe on ground cable at the local disconnect by the air compressor and had a reading of 5 amps.

When you say ground, do you mean Grounded (White) or Grounding (Green)?

..one to the building column, which shows 12 amps on it when I place amp probe on cable?

If you have current on an EGC (Green), this should be fixed first. Check out this recent thread that resolved a similar issue with tracing EGC current to high-z faults.